This invention relates to a process for separating gas mixtures having selectively adsorbable components as for example nitrogen from air, ethylene from nitrogen, and methane and/or carbon monoxide from hydrogen.
Pressure swing adsorption processes are well known for separating gas mixtures having selectively adsorbable components. By way of example one widely used system described in Wagner U.S. Pat. No. 3,430,418 employs four adsorbent beds arranged in parallel flow relationship with each bed proceeding sequentially through a multistep cycle. Since product discharge from a given bed is not continuous, the beds are arranged so that at least one of the four beds is always producing product i.e. one component-depleted gas discharged from the second end. In brief each bed employs an adsorption step in which at least one component of the feed gas mixture is selectively adsorbed from the feed gas introduced at the bed first end and the one component-depleted product gas is discharged from the second end of such bed. The previously described adsorption step is usually performed at the highest pressure of the process and is followed by a first depressurization step in which gas discharged from the bed at progressively lower pressure is used to perform other functions in the process, as for example repressurizing another previously purged bed and/or purging still another bed. This first depressurization step is usually in the same direction i.e. cocurrent, as the feed gas previously flowing through the bed during the adsorption step. After the first depressurization step a final depressurization step usually follows and is most commonly countercurrent to the gas flow during the preceding adsorption and first depressurization steps. During this step gas is usually released at the inlet end and contains desorbate. When depressurization is completed, a purge gas is usually introduced at the second end for countercurrent flow through the bed to desorb and sweep out the desorbate at the inlet end. When purging is completed the bed is repressurized with one component-depleted gas in preparation for return to the previously described adsorption step, and the cycle is repeated.
One disadvantage of the previously described type of pressure swing adsorption process is that multiple beds are required. Another disadvantage is complexity of the piping and multiple valving required to provide the necessary flow switching. Still another disadvantage is that the cycles are relatively long so that the equipment is relatively large and heavy. By way of example, in one such system for air separation the total cycle time for each bed to complete the adsorption through repressurization sequence for air separation in a four bed system is about 240 seconds. This means that the production rate of one component-depleted gas per pound of adsorbent (hereinafter referred to as "adsorbent productivity") is relatively low.
One possible approach to overcoming the previously enumerated disadvantages of multiple bed-relatively long cycle time pressure swing adsorption processes is the rapid pressure swing adsorption process (hereinafter broadly described as "RPSA"). In the RPSA system schematically shown in FIG. 1, a single adsorption bed 10 is provided comprising relatively small particles of adsorbent. The adsorbent particle size used by the prior art may, for example be between 40 and 60 mesh whereas with the aforedescribed multiple bed-relatively long cycle time pressure swing adsorption system (hereinafter referred to as "PSA") the major dimension of individual particles may, for example be 1/16 inch or larger pellets. As used herein, mesh size ranges refer to U.S. standard screens commonly used for sizing small particles. By way of example, "between 20 and 120 mesh" means particles in a size range which pass through a 20 mesh screen and are retained on a 120 mesh screen.
The adsorbent may be a single type of material as for example crystalline zeolite molecular sieve or activated carbon, or may comprise multiple layers or mixtures of different adsorbents for selective removal of particular components from the feed gas. As shown in FIG. 1 the feed gas contains at least two components and is introduced through conduit 11 and pressurized if necessary by compressor 12 followed by a feed surge tank before introduction through valve 13 into the first end 14 of adsorbent bed 10. At least one component of the feed gas is selectively adsorbed and one component-depleted gas is discharged from adsorbent bed second end 15 into conduit 16 having control valve 17 therein. If desired a product surge tank 18 may be provided in conduit 16 upstream valve 17.
In the RPSA system the small adsorbent particles provide the necessary flow resistance to operate the process whereas in PSA this flow resistance is minimized to reduce pressure drop in the adsorbent bed. The aforedescribed flow continues for a predetermined period which will hereinafter be referred to as the "feed gas introduction period" and the one component-depleted gas discharged from the single bed during this period will be termed the "product gas" although it should be understood that the one component desorbate gas thereafter released from first end 14 may be the desired product from a particular feed gas mixture, depending on the consumer's requirements. it is also possible that both gases separated in the RPSA system may be product gases in the sense that each is consumed and not released to the atmosphere.
Following the feed gas introduction period feed valve 13 is closed and exhaust valve 19 in conduit 20 joining the inlet end is opened. During the exhaust period (hereinafter also referred to as the second period) one component-depleted gas within adsorbent bed 10 flows in the reverse direction towards first or inlet end 14. This gas sweeps one component gas towards the first end after such gas has been desorbed from the adsorbent by pressure reduction i.e., the pressure differential between the gas in the bed during the feed gas introduction period and the exhaust pressure. Flow reversal occurs in the adsorbent bed while product is being continuously removed from the second end, and the flow reversal zone moves quickly from the first to the second end during exhaust. Although not essential for all RPSA systems, in some circumstances it may be desirable to provide pump 21 in exhaust conduit 20 to accelerate the reverse outward flow of one component-depleted purging-one component desorbate gas. As will be explained hereinafter typical times for the feed gas introduction period and the second or reverse outward flow period are relatively short and on the order of 0.1 to about 20 seconds. For this reason valves 13 and 19 are preferably the time triggered solenoid type for the relatively small systems described in the ensuing examples. Rotary and poppet valves or other fast-acting valves may be suitable for large systems. Although not essential, RPSA systems often employ a first end flow suspension or time delay step between the feed gas introduction and reverse outward flow, and during such period valves 13 and 19 are both closed but discharge of one component depleted product gas is continued during this period through second end 15.
In the prior art RPSA systems typified by the work of P. H. Turnock and D. E. Kowler at the University of Michigan, equal feed and exhaust times were selected as being most suitable. Unfortunately these prior art experiments involving N.sub.2 -CH.sub.4 separation and air separation resulted in product recoveries (the percent of the one component-depleted gas recovered as product at the bed second end) prohibitively low and not acceptable for commercial use in any type of gas separation even when the feed gas is unlimited, as for example air separation. In any type of pressure swing adsorption system for a given product flowrate the investment cost is the sum of a function of the recovery (reflecting the compressor cost), plus the adsorbent productivity (reflecting the cost of the vessel holding the adsorbent), and other minor items. In general the investment cost is most greatly influenced by the product recovery and this factor represents at least 30% and up to 80% of the investment cost. In general by increasing the product recovery factor the aforementioned investment cost trade-off emphasizes the importance of relatively high product recovery processes. In addition to investment cost, the practioner must consider operating, i.e. power cost. Whereas the latter is unaffected by adsorbent productivity it is directly affected by product recovery. It will be recognized that product recovery may be increased by increasing the feed pressure but this is at the expense of increased power and the cost of power increases may be more rapid than the recovery improvement. In gas separations where the feed gas is available in limited quantity, as for example hydrogen separation and purification from feed gas mixtures containing CH.sub.4 or CO, high product recovery is particularly important because the product and possibly also the exhaust gas must be compressed to substantial pressure such as 200 psig. for its end use. Also for a given adsorbent particle size, bed length and timing cycle, the cross-sectional area of the adsorbent bed is proportional to product gas recovery, i.e. relatively low recovery necessitates a relatively large bed to produce a given amount of product.
An object of this invention is to provide a rapid pressure swing adsorption process having substantially higher product recovery than heretofore attained by the prior art.
Another object is to provide a rapid pressure swing adsorption process providing relatively high adsorbent productivity in addition to the aforementioned high product recovery. Other objects will be apparent from the ensuing disclosure and appended claims.